The shade cloth questions have multiple answers..How tall is the ceiling of your structure..what are you growing....shade loving plants...sun lovers...is water a factor (like draught conditions)..Do the shade trees provide protection from the how afternoon sun? 75% shade is alot
If you have a local brom place where you are maybe they could be the best source of information

I agree with HDD, it depends on what you are growing. If it's Vrieseas, Guzmanias or Cryptanthus, 75% might be okay, but for Neos or Billbergias, go with the 50%.

We have pretty high light intensity where I live, but I find that 60% (black) shadecloth is enough protection even for the Vrieseas.

I don't use it for bromeliads, but in my experience aluminum netting does the best job at reflecting away heat, my greenhouse in full sun only gets about 6-9 degrees F hotter this summer with the aluminet, last year with white cloth it still would easily get 20F hotter in the sun.

I use aluminet also but at different heights over the plants depending what it is..it has the best reflection of light and deflection of heat of any thing I have used. It also protects from frost in the winter and lowered my heating bills on the greenhouse

I use 75% shadecloth (green and black) on sun lovers bromeliads (and even get some leaf sunburn on some of these) because of the low humidity during the dry season (20%-40% humidity). Most nurseries around here use 80% and up (semi-arid).

I guess the question of microclimate is never an easy one. I already have a shadehouse for vriesias, guzmanias, etc. but the other one will be for aechmeas, neo's, epiphyllums & cacti. The height of the roof is about 10-11ft and I'm adding layer of plastic covered with bark chips (kept damp) to the floor for extra humidity. I've arrived at the decision of 50% white for the sides and 75% green for the roof that way the harsh midday sun will be filtered out but the morning sun will be let it. My climate would be similar to patusho's but with less humidity, 30% and we all melt :P so 75% plus the overhanging trees should do the trick and keep nice colouring in the leaves. I notice the price of the aluminet is rather high in Aussieland. It would be rather expensive to do the whole thing with it but a very interesting product all the same. I'd never heard of it until now, thanks for the enlightenment and thanks for all the help with the decisions.

Aluminet may be more expensive, but it is durable, should last a very long time, and most importantly - it might very well be the best material available. It reduces heat buildup very effectively which to me is worth the expense of not having to worry about my plants burning while at the same time not compromising the light quality.

And it makes the greenhouse sparkle in the right light too!

I agree xero ...no shadows even under the benches and still no burned broms

Cool! I'll have to look a bit closer at this aluminet. Amazing stuff if the light intensity allows for no shadows and no scorching too. Could be the answer to all my problems. Thanks all! Forums...what would we do without them :)